The present invention relates to metal-air cells. This invention more particularly pertains to facilitating heat-driven convective airflow in metal-air cells that incorporate air manager technology.
Metal-air cells have been recognized as a desirable means for powering portable electronic equipment, such as personal computers, camcorders and telephones, because such battery cells have a relatively high power output with relatively low weight as compared to other types of electrochemical battery cells. Metal-air batteries include an air permeable cathode, commonly referred to as an oxygen electrode, and a metallic anode separated by an electrolyte. Electrical energy is created with a metal-air battery by an electrochemical reaction.
Metal-air battery cells utilize oxygen from the ambient air as a reactant in the electrochemical process. During discharge of a metal-air battery, such as a zinc-air battery, oxygen from the ambient air is converted at the oxygen electrode to hydroxide, zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy. Metal-air cells utilize oxygen from the ambient air as a reactant, rather than utilizing a heavier material, such as a metal or metallic composition. To operate a metal-air battery, it is therefore necessary to provide a supply of oxygen to the oxygen electrode of the battery.
To preserve the efficiency, power and lifetime of a metal-air cell, it is desirable to effectively isolate the oxygen electrodes and anode of the metal-air cell from the ambient air while the cell is not operating. There are ventilation systems designed to provide the dual functions of providing air to a metal-air cell for power output and isolating the cells during non-use. These known ventilation systems are referred to as air managers. Some air managers only provide air doors that open when power is drawn from the cells and close to attempt to seal the cell housing when the cells are not in use. An important component of successful air managers has been an air mover, such as a fan or an air pump. However, such air movers used in metal-air batteries have been bulky and expensive relative to the volume and cost of the metal-air cells.
While a key advantage of metal-air cells is their high energy density resulting from the low weight of the oxygen electrode, this advantage is compromised by the space and weight required by an effective air mover. Space that could otherwise be used for battery chemistry to prolong the life of the battery must be used to accommodate an air mover. This loss of space can be critical to attempts to provide a practical metal-air cell in small enclosures such as the xe2x80x9cAAxe2x80x9d cylindrical size now used as a standard in many electronic devices. Also, the air mover uses up energy stored in the cells.
U.S. Pat. No. 5,691,074 to Pedicini, entitled xe2x80x9cDIFFUSION CONTROLLED AIR VENT FOR A METAL-AIR BATTERYxe2x80x9d, the entire disclosure of which is incorporated herein by reference, discloses systems for controlling the isolation of one or more metal-air cells from the ambient air while the cells are not operating. In accordance with one example disclosed by Pedicini, a group of metal-air cells are isolated from the ambient air, except for an inlet passageway and an outlet passageway. These passageways may be, for example, elongate tubes. An air moving device circulates air across the oxygen electrodes and forces air through the inlet and outlet passageways to refresh the circulating air with ambient air, so that oxygen is supplied to the oxygen electrodes. The passageways are sized to (i) pass sufficient airflow while the air moving device is operating to enable the metal-air cells to provided an output current for powering a load, but (ii) restrict airflow while the passageways are unsealed and no air is forced therethrough by the air moving device, so that a limited amount of air diffuses through the passageways.
When the air mover is off and the humidity level within the cell is relatively constant, only a very limited amount of air diffuses through the passageways. The water vapor within the cell protects the oxygen electrodes from oxygen exposure. The oxygen electrodes are sufficiently isolated from the ambient air by the water vapor such that the cells have long xe2x80x9cshelf lifexe2x80x9d without sealing the passageways with a mechanical door. The passageways may be referred to as xe2x80x9cisolating passagewaysxe2x80x9d or xe2x80x9cdiffusion limiting passagewaysxe2x80x9d due to their isolating capabilities.
Referring in detail to the isolating passageways described above, these isolating passageways are preferably constructed and arranged to allow a sufficient amount of airflow therethrough while the air moving device is operating so that a sufficient output current, typically at least 50 ma, and preferably at least 130 ma can be obtained from the metal-air cells. In addition, the isolating passageways are preferably constructed to limit the airflow and diffusion therethrough such that the drain current that the metal-air cells are capable of providing to a load while the air moving device is not forcing airflow through the isolating passageways is less than 1 ma per square cm of oxygen electrode surface. Thus, the drain current may be limited to an amount that is smaller than the output current by a factor of at least about 50. In addition, the isolating passageways are preferably constructed to provide an xe2x80x9cisolation ratioxe2x80x9d of more than 50 to 1.
The xe2x80x9cisolation ratioxe2x80x9d is the ratio of the rate of water loss or gain by a cell while its oxygen electrodes are fully exposed to the ambient air, as compared to the rate of the water loss or gain of the cell while its oxygen electrodes are isolated from the ambient air, except through one or more limited openings. For example, given identical metal-air cells having electrolyte solutions of approximately thirty-five percent (35%) KOH in water, an internal relative humidity of approximately fifty percent (50%), the ambient air having a relative humidity of approximately ten percent (10%), and no fan-forced circulation, the water loss from a cell having an oxygen electrode fully exposed to the ambient air should be more than 100 times greater than the water loss from a cell having an oxygen electrode that is isolated from the ambient air, except through one or more isolating passageways of the type described above. In this example, an isolation ratio of more than 100 to 1 should be obtained.
More specifically, each of the isolating passageways preferably has a width that is generally perpendicular to the direction of flow therethrough, and a length that is generally parallel to the direction of flow therethrough. The length and the width are selected to substantially eliminate airflow and diffusion through the isolating passageways while the air moving device is not forcing airflow through the isolating passageways. The length is greater than the width, and more preferably the length is greater than about twice the width. The use of larger ratios between length and width are preferred. Depending upon the nature of the metal-air cells, the ratio can be more than 200 to 1. However, the preferred ratio of length to width is about 10 to 1.
The isolating passageways could form only a portion of the path air must take between the ambient environment and the oxygen electrodes. Each of the isolating passageways may be defined through the thickness of the battery housing or cell case, but preferably they are in the form of tubes as described above. In either case, the isolating passageways may be cylindrical, and for some applications each can have a length of about 0.3 to 2.5 inches (about 0.7 to 6.4 cm) or longer, with about 0.88 to 1.0 inches (about 2.24 to 2.54 cm) preferred, and an inside diameter of about 0.03 to 0.3 inches (about 0.07 to 0.7 cm), with about 0.09 to 0.19 inches (about 0.23 to 0.48 cm) preferred. The total open area of each isolating passageway for such applications, measured perpendicular to the direction of flow therethrough, is therefore about 0.0007 to 0.5 square inches (about 0.0045 to 3.23 sq. cm). In other applications, the isolating passageways each can have a length of about 0.1 to 0.3 inches (about 0.25 to 0.76 cm) or longer, with about 0.1 to 0.2 inches (about 0.25 to 0.5 cm) preferred, and an inside diameter of about 0.01 to 0.05 inches (about 0.025 to 0.013 cm), with about 0.15 inches (about 0.38 cm) preferred. The preferred dimensions for a particular application will be related to the geometry of the passageways and the cathode plenums, the particular air mover utilized, and the volume of air needed to operate the cells as a desired level.
The isolating passageways are not necessarily cylindrical, as any cross-sectional shape that provides the desired isolation is suitable. The isolating passageways need not be uniform along their length, so long as at least a portion of each isolating passageway is operative to provided the desired isolation. Further, the isolating passageways may be straight or curved along their length.
Other exemplary isolating passageways and systems are disclosed in U.S. Pat. No. 5,691,074 and U.S. application Ser. No. 08/556,613, and the entire disclosure of each of those documents is incorporated herein by reference.
Metal-air cells without air managers have found limited commercial use in devices, such as hearing aids, which require a low level of power. In these cells, the air openings which admit air to the oxygen electrode are so small that the cells can operate for some time without flooding or drying out as a result of the typical difference between the outside relative humidity and the water vapor pressure within the cell. However, the power output of such cells is too low to operate devices such as camcorders, cellular phones, or laptop computers. Enlarging the air openings of a typical xe2x80x9cbutton cellxe2x80x9d would lead to premature failure as a result of flooding or drying out.
Metal-air cells without an air mover must rely on passive airflow to provide oxygen to the air electrode. However, the passive airflow is not sufficient to power many electrical devices. Also, in known metal-air batteries, the direction of any passive airflow will be dependent upon the orientation of the metal-air cell because hotter air naturally rises while cooler air falls. Thus, creating convective airflow is more easily accomplished when the metal-air cell is only intended for use in a single orientation. There is a need for an orientation independent system and method for facilitating convective airflow in a controlled manner over the air electrodes of a metal-air battery at rates sufficient to power electronic, without a mechanical air mover. There is a further need for a system and method of combining air manager technology with cylindrical metal-air cells in an orientation independent manner.
The present invention alleviates or solves the above-described problems in the prior art by facilitating convective airflow through an orientation independent metal-air power source. The present invention seeks to provide a convective air manager for a metal-air battery that is capable of creating a xe2x80x9cchimney-effectxe2x80x9d within the metal-air battery. Enhanced convective airflow through the metal-air battery eliminates the need for utilizing a mechanical air mover in an air manager.
Generally described, the present invention provides a housing for enclosing at least one metal-air cell. A path of convective airflow passes through the housing. The path communicates with the exterior of the housing during discharge of the metal-air battery and at least a portion of the path has a significant vertical component in every orientation of the metal-air battery. At least a portion of the path is heated during discharge of the metal-air cell.
In accordance with one embodiment of the invention, a housing encloses at least one metal-air cell. The housing includes an air cavity at each end with a central air passageway therebetween. Each cavity includes one or more resistance elements which selectively heat the air within each cavity. A temperature differential is created between the heated air and the cooler air elsewhere in the housing which causes convective airflow between the cavities through the central passageway. A plurality of isolating passageways are provided which extend through the walls of the housing to the cavities for inlet and outlet airflow.
In accordance with another embodiment of the invention in a cylindrical cell, the central passageway is prismatic and is defined between a pair of planar oxygen electrodes. The cavities are disk-shaped and have a radius larger than a width of the central passageway between the pair of oxygen electrodes. Alternatively, the central passageway may be cylindrical and defined by a coaxial cylindrical oxygen electrode. In such case, the cavities would have a larger outer periphery than an outer periphery of the cylindrical central passageway. In either case, the resistance elements are preferably positioned in the outer periphery of each cavity, and may be arcuate.
In accordance with another aspect of the present invention, a centrally located gravity switch is provided in the housing of the metal-air battery. The switch includes a plurality of contact pairs which are coupled to the resistance elements. Each contact pair is associated with a particular orientation of the metal-air battery and a particular resistance element to be heated when the battery is in that orientation. A conductive ball is also provided for completing the circuit between a contact pair to allow at least a portion of one of the resistance elements to be heated.
Alternatively, these objects are accomplished in a prismatic metal-air power source having a substantially L-shaped air plenum defined by the interior surfaces of the housing and the enclosed metal-air cell. A plurality of isolating passageways connect the air plenum without outside air. The isolating passageways are oriented in a manner that facilitates convective airflow through the housing and across the oxygen electrode of said metal-air cell Heating may be applied selectively to areas of the plenum to drive the convective flow.
In any of the foregoing embodiments, the one option for providing heat to enhance convective air flow is to heat the isolating passageways to create a chimney effect within the heated isolating passageway.
Accordingly, an object of this invention is to provide a ventilation system for metal-air batteries that overcomes the aforementioned inadequacies of prior art ventilation systems.
Still another object of the present invention is to provide a structurally simple and economical power source utilizing convective airflow that functions independent of its orientation.
Yet another object of the present invention is to provide a ventilation system for a metal-air power source wherein the operation of the power source is transparent to the user. That is, in order to operate the ventilation system of the present invention, no action on part of the user is required.
The foregoing has broadly outlined some of the more significant objects and features of the present invention. These should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or by modifying the disclosed embodiments. Accordingly, other objects and a more comprehensive understanding of the invention may be obtained by referring to the detailed description of the preferred embodiment taken in conjunction with the accompanying drawings, in addition to the scope of the invention defined by the claims.